1. Field of the Invention
The present invention relates generally to nebulizers for use in analytical spectrometry such as inductively coupled mass spectrometry, and more generally to direct injection nebulizers.
2. Description of the Related Art
In inductively coupled plasma (ICP) spectrometries, test solutions are typically introduced into the plasma in the form of an aerosol. The combination of a pneumatic nebulizer (PN) with a spray chamber is primarily used in ICP spectrometries because of its simplicity and low cost. However, this arrangement suffers from low analyte transport efficiency (typically 1-2%) and high sample consumption (typically 1-2 mL/min). A simple, low-consumption, highly efficient nebulizer is often required in chromatographic applications and also for the direct analysis of semiconductor, biological, forensic, or toxic materials. In these and other cases, the sample is limited, expensive, or hazardous, and it may contain a large fraction of organic solvents that can substantially alter the plasma characteristics. Presently available microflow nebulizers include pneumatic devices such as the high-efficiency nebulizer (HEN), the microconcentric nebulizer (MCN), the direct injection nebulizer (DIN), and the oscillating capillary nebulizer (OCN), as well as piezoelectrically driven devices such as the microflow ultrasonic nebulizer (.mu.-USN) and the monodisperse dried microparticulate injector (MDMI). Among these devices, the DIN has received significant attention, partly because in this device the nebulizer is an integral part of the ICP torch, and thus 100% of the aerosol is presented to the plasma. This attribute along with the low internal dead volume (&lt;2 .mu.L) of the DIN leads to several other benefits, namely: low memory effects, rapid response times, and good precision. These characteristics are particularly important for interfacing liquid chromatography equipment to ICP-based instruments.
Unfortunately, the setup required for the DIN is more expensive and complex than the conventional pneumatic nebulizer-spray chamber arrangement. The DIN also requires a high pressure pump for sample delivery. Further, because no spray chamber is used, relatively large, high-velocity droplets with broad size-velocity distributions are introduced into the plasma. These drawbacks result in reduced sensitivity, impaired precision, and increased matrix effects.
The HEN-spray chamber combination offers detection limits (ppt) at solution uptake rates of less than 100 .mu.L/min (similar to conventional PNs operated at 1-2 mL/min), and the primary aerosol produced by the HEN exhibits droplet-size distributions that are smaller and narrower than those reported for the DIN (see S. C. K. Shum, S. K. Johnson, H- M. Pang, and R. S. Houk, "Spatially Resolved Measurements of Size and Velocity Distributions of Aerosol Droplets from a Direct Injection Nebulizer," Applied Spectroscopy, vol. 47, pp. 575-583, 1993). However, the HEN is configured to be used with a spray chamber, and does not permit direct injection of the aerosol into the plasma.
Accordingly, there is a need for a simple, low-cost device that produces relatively small droplet sizes in a relatively narrow droplet size distribution for direct injection of microliter quantities of test solutions into the plasma.